1. Field of Invention
The present invention relates to metallic material or film having a fluorinated surface layer, and a fluorination method of the metallic material or film. More particularly, the present invention provides fluorinated metal, on the top surface of which a thick fluoride layer greately enhances the corrosion resistance. The metal may be in any form capable of forming the fluoride layer thereon. The metal may be monolithic material or film formed on the substrate.
It is, particularly, intended by the inventive technology that the metallic material or film be used in a production apparatus of semiconductor devices and the like, so as to realize extremely advantageous corrosion performance against halogen-based corrosive gases, such as chlorine-, fluorine- or bromine-based gases.
2. Description of Related Art
In the production process of semiconductors, halogen-based, reactive and strongly corrosive special gases such as hydrogen chloride (HCl), boron trichloride (BCl.sub.3), fluorine (F.sub.2), nitrogen trifluoride (NF.sub.3), chlorine trifluroride (ClF.sub.3) and hydrogen bromide (HBr) are used. These gases are easily hydrolyzed by the presence of water in the environment, thus generating hydrochloric acid, hydrofluoric acid, hydrobromic acid and the like. The constructional metallic material or film of a valve, coupling, pipings, reaction chamber and the like for treating these gases is easily corroded and problems incurr.
In addition, these corrosive gases are converted to plasma or are thermally decomposed. They are decomposed to active atom species and are used for etching the oxide film or metallic film and are used for dry-cleaning the reaction chamber as well. Recently, in the production of super ULSIs and the production process of liquid crystals, the amount of such gases used has abruptly increased. The highest quality of cleanliness and corrosion performance is required for the plant materials, such as the surface of a reaction chamber.
In addition, since fluorine gas is mixed with inert gas (krypton, neon, argon) and is oscillated in the field of an excimer laser, extremely strict corrosion performance is required for the material surface of a plant against the fluorine radicals.
Electrolytically polished stainless steel SUS 316L can allegedly solve the above described problems and is usually used. Such stainless steel is subjected to baking at 250.degree. C. prior to use. However, the corrosion resistance of stainless steel does not satisfactorily meet the requirements. Various nickel-based alloys have, therefore, been employed with halogen gas such as gaseous hydrechloric-acid at high temperature.
However, various problems are left unsolved in these parts as well.
First, the metallic material itself is expensive. The formability of the metallic material into parts of a plant is poor. This finally leads to considerable cost increase of the plant. Furthermore, the corrosion resistance is attained only by a limited composition. For example, Hastelloy-C (Ni--Cr--Mo--W alloy) exhibits extremely improved corrosion resistance against the oxidizing acid and also exhibits improved corrosion resistance against even the reducing acid, such as hydrochloric acid, when used at room temperature. In addition, Hastelloy-C exhibits remarkable resistance against pitting corrosion and crevice corrosion. However, it is pointed out that, since the corrosion resistance of Hastelloy-C is poor against the fluorine gases and the fluorine radicals mentioned above, Hastelloy-C is not usable.
A large amount of research has heretofore been made with regard to the passivation technique with the use of fluorine gas. It is known that a passivation film in the order of angstroms is formed on the metallic surface of nickel so as to create the corrosion-resistance function.
In addition, Japanese Unexamined Patent Publication (kokai) No. 2-263972 is related to the invention entitled "Metallic Materials with Fluorinated Passivation Film Formed Thereon and Apparatus with the Use of Such Metallic Materials". The publication discloses the metallic material or film, on which the passivation film is formed, and an apparatus, in which the metallic material and coating are used. In this publication, a passivation film is formed by means of fluorine gas on the metal which is at least one selected from nickel, nickel alloy, aluminum, aluminum alloy, copper, copper alloy and chromium, among the metals. The corrosion resistance disclosed is of improved quality. However, the film formed is of from 1000 to 3000 angstrom thick and hence ultra thin. The surface state of aluminum, stainless steel, copper and nickel plates to be fluorinated in this publication is a polished surface.
In addition, Japanese Unexamined Patent Publication (kokai) No. 2-175855 is related metallic material or film, on which the fluorinated passivation film is formed, as well as an apparatus, in which the metallic material and film are used. The publication discloses a process for forming on the surface of stainless steel a mixed fluoride layer of iron fluoride and chromium fluoride. A fluorinated passivation film in the order of sub-micron thickness as well as the material with such film are disclosed. Improved corrosion resistance is disclosed. Thickness of the film formed is 4000 angstrom and is ultra thin. Incidentally, the polished SUS316L sheet is subjected to the fluorination.
Since the fluorinated passivation films formed in the above publications are of approximately 4000 angstroms or less in thickness, they are easily removed by flaws, friction and the like. It is, therefore, difficult to say that the films are appropriate as the material of production apparatuses of semiconductor devices from the viewpoints of durability and longevity.
The present invention aims to solve the problems involved in the prior art described above. The conventional passivation techniques are characterized in that the material surface is cleaned by polishing and the like and is then fluorinated to passivate it. It was discovered that, when the surface is oxidized to passivate it and is then fluorinated, surprisingly, not only the passivated and oxidized surface exhibits no hindrance to the fluorination, but also a rather thick fluorinated layer can be formed.